Evidence for a role of Anopheles stephensi in the spread of drug- and diagnosis-resistant malaria in Africa

Abstract

Anopheles stephensi, an Asian malaria vector, continues to expand across Africa. The vector is now firmly established in urban settings in the Horn of Africa. Its presence in areas where malaria resurged suggested a possible role in causing malaria outbreaks. Here, using a prospective case-control design, we investigated the role of An. stephensi in transmission following a malaria outbreak in Dire Dawa, Ethiopia in April-July 2022. Screening contacts of patients with malaria and febrile controls revealed spatial clustering of Plasmodium falciparum infections around patients with malaria in strong association with the presence of An. stephensi in the household vicinity. Plasmodium sporozoites were detected in these mosquitoes. This outbreak involved clonal propagation of parasites with molecular signatures of artemisinin and diagnostic resistance. To our knowledge, this study provides the strongest evidence so far for a role of An. stephensi in driving an urban malaria outbreak in Africa, highlighting the major public health threat posed by this fast-spreading mosquito.

Fig. 1. Global distribution of An. stephensi and the study location.

a, The global distribution of An. stephensi where it is native (blue) and invasive (red) is shown, together with sporozoite infection detection outcomes where it was found infected and not infected with P. falciparum (Pf) and P. vivax (Pv). Sites where An. stephensi was observed but mosquitoes were not tested for the presence of sporozoites are also shown (not determined). Settings where dedicated entomological surveillance did not detect An. stephensi mosquitoes are indicated by gray circles (negative). b, The locations of case (red) and control (green) households/dormitories surveyed in this study are shown, together with water storage containers (black), water treatment plant (in the university campus), health facilities (H) and Butiji River in Dire Dawa City. Source: the global map (a) was modified on the basis of the malaria threats map of the WHO. Source data

Fig. 2. Temporal trends in malaria burden and parasite density distributions in Dire Dawa.

a,b, Malaria trends using DHIS2 data (a) are shown, with the prevalence and odds of detecting additional infections in close contacts of cases compared with controls in Dire Dawa, separately for all close contacts, contacts in the city and the university (b). ORs were obtained from univariate logistic regression, with diagnostic test results as outcome and site as predictor. Univariate logistic models were fitted for each diagnostic test. ORs are shown on a log₁₀ scale (x axis), together with their 95% CI bars and respective P values (estimated from Wald test). Numbers to the right of the forest plot indicate the proportion of positive cases by respective diagnostic test (color coded and embedded in the figure) among control and index household/dormitory members. c–e, Parasite density per microliter distributions and their respective averages determined by 18S-based qPCR among HRP2-based RDT-positive (n = 113) and RDT-negative (n = 88) infections (c) and microscopy-positive (n = 129) and microscopy-negative (n = 71) infections (d) are shown, together with distribution among index cases (n = 99), contacts of index cases (n = 61), controls (n = 14) and contacts of controls (n = 27) (e). Source data

Fig. 3. Spatial distribution and clustering of P. falciparum parasites and An. stephensi.

a–c, Statistically significant evidence for global spatial clustering of household P. falciparum infections prevalence (a) and An. stephensi mosquitoes (b) and an overlap between the two (c) were observed. Eleven clusters of households were found (A) in the city (P < 0.05) by one-sided local Anselin Moran’s I test (pseudo P values calculated from 9,999 random permutations): high–high (n = 6) whereby households had high P. falciparum prevalence, low–low clusters (n = 5) whereby households had low P. falciparum prevalence, and high–low outlier clusters (n = 2) whereby high P. falciparum prevalence households were surrounded by low P. falciparum prevalence households, or vice versa. Locations of An. stephensi mosquitoes found infected (n = 3) are shown in dark-red circles and triangles (b). d,e, A map displaying case incidence colored by genetic cluster (lineage 1 in blue and lineage 2 in red) are shown along with timelines that cases were identified (d) and their spatial distribution (e). Source data